1. Field of the Invention
The present invention relates to a manufacturing method of a substrate of a compound formed by a group III element and nitrogen (hereinafter referred to as a nitride), which is expressed by AlxGayIn1-x-yN (0≦x<1, 0<y≦1).
2. Description of the Background Art
While a crystal such as GaN, AlGaN, InGaN, or InN is often epitaxially grown as a thin film over a substrate such as sapphire or SiC, it is difficult to manufacture a thick free-standing substrate. Even when a free-standing substrate is obtained, often, dislocation density is high and the quality is low. At present, there are no large nitride free-standing crystal substrates having low dislocation density and high quality.
As a substrate for a blue light emitting element, a sapphire substrate is widely employed. A blue or green light emitting element (light emitting diode (LED) or laser diode (LD)) is manufactured by forming n-type and p-type thin films of GaN or InGaN on a sapphire substrate (α-Al2O3). The sapphire substrate is readily available, and physically and chemically stable and a GaN or InGaN thin film can be grown thereon.
The sapphire substrate is disadvantageous in that it is an insulator and has no cleavage. Sapphire is not threefold rotational symmetric and therefore does not have a clear cleavage plane. It cannot be separated into chips by natural clearage after multiple light emitting element units are formed thereon. Mechanical cutting by dicing is required. This reduces the yield.
Since it is an insulator, an n electrode cannot be formed on the rear surface. Both n and p electrodes have to be formed on the upper surface, whereby the required chip area is increased, the etching steps are increased and the wires bonded are doubled. Since a current flows laterally in the n layer, the n layer must be thick.
Since the lattice mismatch is great between sapphire and gallium nitride (GaN), GaN or InGaN epitaxially grown on the sapphire substrate contains dislocations of high density. Therefore, it is a thin film of low quality. A commercially available GaN thin film formed on a sapphire substrate contains dislocations in a density of not less than 1×109cm−2.
Because of such disadvantages, instead of the sapphire substrate, a GaN substrate, which is identical to the thin-film composition, is desirable. However, GaN does not melt even under high temperatures. The dissociation pressure of nitrogen under high temperatures is too high. The crystal growth cannot be attained by the Bridgeman method or the Czochralski method, which are methods of growing a crystal from melt.
Accordingly, instead of growth from liquid phase, vapor phase growth method is employed. Originally, vapor phase growth method is employed for manufacturing a thin film having a thickness of not more than about 1 μm, and not for a thick crystal such as a substrate. For manufacturing a GaN thin film, four vapor phase growth method, namely, the HVPE (Hydride Vapor Phase Epitaxy) method, the MOCVD (Metal Organic Chemical Vapor Deposition) method, the MOC (Metal Organic Chloride) method, and the sublimation method are possible.
The HVPE method is performed as follows. A Ga boat containing Ga melt is placed in an upper space of a hot-wall type reactor. A ground substrate (sapphire, SiC or the like) is placed on a susceptor in a lower space, and heated. From the above, H2+HCl gas is blown in the Ga boat, to produce GaCl. H2+NH3 gas as supplied from the above and GaCl are reacted, to produce GaN, which is deposited on the ground substrate. H2 is the carrier gas. The Ga material is metal Ga, and it is heated and used as a melt. The material of nitrogen is NH3 gas. There is an advantage that carbon is not mixed into the generated GaN thin film, since the material gas does not contain carbon.
The MOCVD method is performed as follows. In a cold-wall type reactor, an organic metal material of Ga such as TMG (trimethyl gallium) and ammonia (NH3) together with hydrogen gas are blown in a heated ground substrate, to produce GaN, which is deposited on the ground substrate. On the heated substrate, GaN is produced by the one-stage reaction of TMG+NH3. This is the most frequently used method as a method for growing a GaN thin film. Thin-film growth for a blue-light emitting element is mostly performed by this method. However, there is a problem that the method requires a great amount of NH3 gas and the yield is low. While the method is suitable for thin-film production, it is not very suitable for manufacture of a thick substrate crystal. Furthermore, since the material containing carbon is used, there is a problem that the carbon mixes into the GaN thin film.
The MOC method is performed as follows. In a hot-wall type reactor, TMG and HCl gas are reacted, to firstly produce GaCl. GaCl is caused to flow downstream and reacted with NH3, to produce GaN, which is deposited on the heated ground substrate. It is similar to the MOCVD method above in that TMG is used as the Ga material. It is similar to the HVPE in that the two-stage reaction is employed, i.e., GaCl is firstly produced and then reacted with NH3. Herein also the material includes carbon. However, since GaCl is firstly synthesized and then reacted with NH3, mixture of carbon into GaN can be prevented. This is the advantage of the MOC method over the MOCVD method. This method also consumes a great amount of the material gas because TMG gas is used, and the yield is poor.
The sublimation method is performed as follows. Polycrystal GaN is used as the material. In a reactor, solid GaN material and a ground substrate are placed in separate locations, and heated in such a manner that the solid GaN material is heated to a higher temperature. Thus, the temperature gradient is created, whereby the sublimated GaN flows toward the ground substrate and deposits on the substrate having a lower temperature.
When a GaN thin film is grown on a ground substrate such as a sapphire, many dislocations occur. The dislocations extend upward, and hardly eliminated.
[1. ELO Method]
The first method used for reducing dislocations in a GaN thin film was the ELO (Epitaxial Lateral Overgrowth) method. On a sapphire ground substrate, an SiO2 film is applied, and small windows each measuring about 0.5 μm to 2 μm are opened in a staggered manner. Thus, a mask is formed in which exposing portions (the windows) are far smaller than covering portion (the SiO2 film). This is referred to as an ELO mask. Since growth of GaN is started from each isolated exposing portion, many isolated crystal grains each having a shape of truncated cone are produced. When the crystal rises over the edge of the covering portion, the growth proceeds laterally. The dislocations also extend laterally. The dislocation density becomes low over the exposing portion. Crystals laterally extending from adjacent windows collide and integrated on a bisector. Here, part of the dislocations cancel each other and are eliminated. At the integrated portion, dislocations survive and start growing upward.
This is a phenomenon that proceeds for a thickness as great as the interval of the windows (about 1 μm to 3 μm). This is effective for a thin film because of the thinness. However, thereafter, upward C-plane growth starts. The dislocations is not eliminated but also extend upward. When the crystal is grown thickly, the dislocations are dispersed and ultimately attaining a high dislocation density of not less than 109cm−2. The ELO method, which merely cancels dislocations by lateral growth at the early stage of the growth, is not useful for reducing dislocations in a thick substrate crystal. However, in some cases, the present invention employs the ELO mask for part of the exposing structure in an additionally manner.
GaN is hexagonal system, wherein a-, b-, and c-axes are not equal. It has a threefold rotational symmetric structure around c-axis. When growing GaN on a foreign material substrate (sapphire, SiC), a ground substrate of which surface is the plane with threefold rotational symmetry is used. Accordingly, when the ground substrate is sapphire or SiC, it is a crystal having the surface of C-plane. When GaN is grown thereon, it grows in c-axis direction. Normally, the growth is carried out so that the surface becomes a flat (mirror surface) C-plane, and therefore the growth is referred to as C-plane growth. On C-plane sapphire ground substrate, GaN crystal of which surface is C-plane is grown.
[2. Random Facet Growth Method]
Japanese Patent Laying-Open No. 2001-102307 (Patent Document 1) discloses the following. Growth conditions are so set intentionally that they do not satisfy the mirror surface growth conditions, whereby multiple three-dimensional facet pits are naturally produced on the crystal surface, and grown while ensuring that facets are not buried. Dislocations are gathered at the bottom of the facet pits, so that they are reduced in the other portions. Thus, low dislocation density in the other portions is attained. Since the growth proceeds in c-axis direction, C-plane is the average growth plane. As used herein, a facet refers to a plane having low Miller indices except for C-plane.
A facet pit is a funnel-like recess having a shape of regular hexagonal cone or regular dodecagonal cone. In a regular hexagonal cone facet, six facet planes {1-101} or {11-22} having different Miller indices are gathered to form one pit.
A regular dodecagonal cone is formed by a combination of twelve facet planes {1-101} and {11-22}.
Each facet plane of a facet pit has a normal that is directed inward. The crystal grows in the direction of the normal. The dislocations extend in the growth direction. Accordingly, the dislocations extend inward and gather at the boundaries of the facets. Then, they slide along the boundary planes and gather at the bottom of the pit. A specific portion where the dislocations are gathered is created at the pit bottom. This is a portion where defects are gathered. While the pit bottom has high dislocation density, the other portions are deprived of dislocations and therefore have low dislocation density. At average, dislocation density is reduced to about 2×107cm−2.
This scheme is tentatively referred to as “the random facet method”, as portions where dislocations gather (pit bottoms) are randomly produced on the crystal plane. This is the first of facet growth methods made by the present inventors. The facets naturally occur. The locations where pits are produced are random and cannot be controlled. A device must be manufactured selecting the portion with fewer defects. However, occurrence of the defects are determined by accidents. Further, since nothing enclose the dislocations, the dislocations once gathered may spread. Still further, as to the hexagonal cone pit, planar defects may occur in the angle of radiation of 60° immediately below the boundary lines. There have been such problems.
[3. Dot Mask Facet Growth Method]
Japanese Patent Laying-Open No. 2003-165799 “Single Crystal GaN Substrate, Method of Growing Same and Method of Producing Same” (Patent Document 2) is the second of the facet growth methods made by the present inventors. On a ground substrate, isolated dot-like masks (SiO2) each having a diameter of about 10 μm to 40 μm are regularly arranged in a matrix with a pitch of about 100 μm to 400 μm. GaN is grown thereon by a vapor phase growth method. While being referred to as the “mask” herein, this mask has narrow covering portions and wide exposing portions contrary to the ELO mask (which has wide covering portions and small windows).
In addition, the pitch of the mask arrangement is not some Am but wider, i.e., about 100 μm to 400 μm. The phenomenon occurring on the masks is also contrasting. The crystal growth proceeds faster in the exposing portion of the ground substrate, while the growth proceeds slower in the covering portions (SiO2). Accordingly, a facet pit where facets converge is produced at the mask-covering portion. The pit becomes greater as the growth proceeds, having its bottom on the covering portion. That is, the location where the facet pit is produced corresponds to the mask-covering portion. The location above the mask-covering portion corresponds to the pit bottom.
As the growth proceeds, dislocations moves in the normal direction of the facet planes, and reach facet boundaries. From the facet boundary, the dislocations moves to the facet bottom. The dislocations gather at the facet bottom, to form a closed defect gathering region H there. The facet bottom means the location above the mask. On the mask-covering portion, closed defect gathering region H is formed. Closed defect gathering region H is slightly smaller than the mask. By the mask, the position and size of closed defect gathering region H can be determined. The other portion of the facet slanting plane becomes a low defect single crystal region Z.
As the masks are formed as isolated dots and the facets are formed in a fashion of isolated circles, there is a portion that is not covered by the facets and where C-plane growth proceeds. While this portion is also low in dislocation density and it is a single crystal, it is different in various aspects from low defect single crystal region Z that is grown continuously from the facet plane. Therefore, this portion is referred to as a single crystal low dislocation extra region Y. In this method, the position of closed defect gathering region H can be determined from the beginning by the mask position. The size of closed defect gathering region H can also be determined by the size of the mask.
Furthermore, closed defect gathering region H formed continuously from the mask-covering portion is closed. Once the dislocations are accumulated in closed defect gathering region H, they will not spread by the progress of the crystal growth. This is why the term “closed” is used. Being different from Patent Document 1, dislocations gather at the bottom. Accordingly, no planar defects are produced under the boundaries of the facets. While the dislocation density is 106 to 107 cm−2 at low defect single crystal region Z near closed defect gathering region H, at a portion away from closed defect gathering region H by 100 μm, the dislocation density is reduced to 104 to 105 cm−2. Since the mask-covering portions being the origin of closed defect gathering regions H are present in a manner of isolated point “dots”, the present method is referred to as the dot mask type herein.
[4. Stripe Mask Facet Growth Method]
Japanese Patent Laying-Open No. 2003-183100 “Single Crystal GaN Substrate, Method of Growing Single Crystal GaN, and Method of Producing Single Crystal GaN Substrate” (Patent Document 3) discloses the following. On a ground substrate, a plurality of masks (such as SiO2) parallel to one another at regular intervals are formed, and GaN is grown by a vapor phase growth method thereon. It is different from Patent Document 2 in that the mask has a plurality of covering portions provided at regular intervals that are linearly parallel and that are continuous from one side to the other side of the ground substrate. It is the same in that the covering portions are smaller in the area than the exposing portions. However, while Patent Document 2 discloses the mask in which isolated covering portions are arranged regularly in a matrix, Patent Document 3 discloses the mask in which linear covering portions are arranged regularly in parallel to one another. The crystal is grown by a vapor phase growth method of the HVPE method or the MOCVD method.
In case of the HVPE method, in epitaxial growth, NH3 partial pressure is 30 kPa and HCl partial pressure is 2 kPa (see paragraphs 229 and 291 of Patent Document 3). Alternatively, NH3 partial pressure is 20 kPa or 25 kPa and HCl partial pressure is 2.5 kPa or 2.0 kPa (see paragraphs 311, 335 and 371 of Patent Document 3).
Crystal growth precedes in the exposing portions of the ground substrate, while crystal growth delays in the covering portions. Therefore, as GaN is grown, parallel mountain-shaped facets are produced, in which the covering portions are the valleys and the centers of the exposing portions are ridges. Parallel facet grooves, but not pyramid pits, are produced. A repetitive structure of parallel ridges and volleys similar to mountains is produced. Along the growth, dislocations slide downward on the sides of the facets, and collected at the valleys of the facets. The valleys are located immediately above the mask-covering portions. The location immediately above the covering portions are the regions where defects gather. Since there are a number of mask-covering portions in parallel to one another, the regions where defects gather are also produced in parallel to one another. The defects do not gather at a pit bottom, but they gather at the bottom of a V-groove. This is referred to as crystal defect gathering region H. It is not closed as closed dislocation gathering region H in Patent Document 2. Therefore, the term “closed” is not used.
Since the portions below facet slanting planes are deprived of dislocations, the portions except for dislocation gathering region H attain a low dislocation density. The slanting planes of a facet are determined, and they are significantly steep. Since the covering portions are parallel to one another, the portions where C-plane growth proceeds (single crystal low dislocation extra region Y) can completely be reduced to zero. Then, a repetitive structure HZHZH . . . of parallel low defect single crystal region Z and dislocation gathering region H is produced. While it depends on the Miller indices of the facet planes, as the slanting angle of a facet plane is about 50 degrees, the crystal becomes a parallel mountain shape having significantly high ridges and low valleys. As it is transparent, it has a shape in which a number of triangle columnar prisms are arranged side by side in parallel to one another. When a portion where C-plane growth proceeds is left at the intermediate portion of low defect single crystal region Z, a repetitive structure of HZYZHZYZH . . . is produced.
When light emitting devices are to be manufactured thereon, they may be formed on regions Z or Y where the defect density is low. In Patent Document 3, width h of defect gathering region H is 1 μm to 200 μm, while the width of the combination of low defect single crystal regions Z and C-plane growth region Y (y+2z) is 10 μm to 2000 μm. The spatial period (pitch p) of the structure of HZHZHZ . . . or HZYZHZYH . . . arrangement is 20 μm to 2000 μm. Since a facet plane has steep slanting angle of about 50 degrees, the height of the ridge is the degree of pitch p. For example, when p=2000 μm, the height of the ridge portion of the crystal will be about 2 mm and many concave and convex portions are present in the plate. The appearance is significantly different from a crystal obtained by normal vapor phase growth. Since the crystal immediately after the growth has great concave and convex portions, it is ground and polished to be a flat and smooth substrate crystal.
The crystal processed to be the flat and smooth substrate has parallel regions Z or ZYZ which have low dislocation density and which are single crystal. Accordingly, it has a structure suitable for manufacturing many devices on low defect single crystal regions Z. This is because low defect single crystal regions Z each has a width of 2000 μm, at a maximum. As 2000 μm corresponds to 2 mm and the size of a light emitting device chip is often 300 μm to 500 μm square, about three to five chips can be arranged next to each other in one line of low defect single crystal region Z.
As represented by GaN substrates, there are no large-size nitride semiconductor (AlGaInN) substrates having low dislocation density. While nitride semiconductor devices are formed on a foreign material substrate (sapphire, SiC), there are problems of cleavage, insulation, high defect density and the like. They are great obstacles in fabricating the nitride semiconductor devices.
In particular, as substrates for mass production, the substrates are required to have a great area, low defect density, and an excellent quality. To this end, there have been many attempts of fabricating low dislocation density nitride substrates of large size. However, it is not yet in actual use.
As substrates for electronic devices, desirably the dislocation density is not more than 1×106cm−2, or not more than 1×105cm−2, if possible. Therefore, the GaN substrate of the present inventors (Patent Document 3: Japanese Patent Laying-Open No. 2003-183100) in which dislocations are gathered to defect gathering region H so that surrounding portions become regions of low dislocation density is desirable.
In the GaN substrate based on Patent Document 3 of the present inventors, the dislocation density (at region Z) of 1×106cm−2 can stably be controlled. However, since low defect single crystal region Z and crystal defect gathering region H must be alternately present, it is difficult to form low defect regions having a large area, which is required for creating electronic devices and the like.
Patent Document 3 is the third of the facet growth methods made by the present inventors, and it is the most relevant to the present invention. In the stripe mask facet growth method of Patent Document 3, on a ground substrate, stripe mask patterns in which covering portions and exposing portions are regularly arranged in parallel to one another are arranged. Linear ridges and V-grooves (valleys) made of facet planes are formed thereon. While maintaining them, the facet growth of GaN is allowed to proceed. Defect gathering region H is created at the bottom of V-groove made of facet planes. Dislocations are gathered thereto, whereby dislocation density in surrounding low defect crystal region Z and C-plane growth region Y is reduced. The crystal growth provides a stripe-patterned plate having ridges and valleys arranged side by side, with concave and convex portions. By polishing the plate to abrade the ridges, a flat substrate is provided. In the GaN crystal processed to be the substrate, the width 2z of low defect single crystal regions Z, the combined width 2z+y of low defect single crystal regions Z and C-plane growth region Y, or pitch p is narrower than 2000 μm.
In the GaN substrate crystal according to Patent Document 3, stripe-like defect gathering regions H and low defect density regions are alternately present. Devices cannot be provided on defect gathering regions H. The devices can be provided only on low defect single crystal regions Z. However, the area of low defect density regions (Z and Y) is small, posing a problem in using the substrate as a GaN substrate for LEDs or electronic devices. Wider low defect single crystal regions Z are desired.
In the defect gathering growth method according to Patent Document 3, defect gathering region H is at the bottom of a groove, and low defect single crystal regions Z made by facet planes are formed around it. Defect gathering region H is essential for converting the other portions into low defect single crystal regions Z. Without defect gathering region H, low defect single crystal regions Z are not formed. If the distance (z or 2z+y) between defect gathering region H and defect gathering region H is increased for enlarging low defect single crystal regions Z, problems such as irregularity of the growth plane, occurrence of crystal defects and the like arise. That is, the above-described conditions of epitaxial growth are not applicable to the facet growth with great pitch p or width z of low defect single crystal region Z.
In the epitaxial growth of GaN according to the HVPE method of Patent Document 3, NH3 partial pressure is 30 kPa and HCl partial pressure is 2 kPa (see paragraphs 229 and 291 of Patent Document 3). Alternatively, NH3 partial pressure is 20 kPa or 25 kPa and HCl partial pressure is 2.5 kPa or 2.0 kpa (see paragraphs 311, 335 and 371 of Patent Document 3). Thus, the most frequently employed condition in Patent Document 3 is the typical condition wherein NH3 partial pressure is 30 kpa and HCl partial pressure is 2 kPa. However, with such a condition, the facet growth cannot be stably performed on the ground substrate with a stripe mask having wide pitch p.